IMS centralized services (IMS Centralized Services, ICS) support use of circuit switched (Circuit Switched, CS) access to bear a media stream to implement Internet Protocol multimedia subsystem (IP Multimedia Subsystem, IMS) services. A service centralization and continuity (Service Centralization and Continuity, SCC) application server (Application Server, AS) is an application server that supports the ICS and is used to act as an agent of a user equipment (User Equipment, UE) to perform IMS session control, for example, being triggered by the UE to initiate an IMS session.
The ICS working group already completes the standardization of the framework, and implementation of the standardization of the ongoing protocol relates to stipulating a new protocol, that is, an ICS control protocol (or known as an I1 protocol). The I1 protocol is an application protocol on the I1 interface. The I1 interface is a new interface introduced by the ICS working group. The I1 interface is located between the UE and the SCC AS and used to transfer IMS service control signaling. The I1 interface is based on CS access, for example, the application protocol message of the I1 interface is transferred through unstructured supplementary service data (Unstructured Supplementary Service Data, USSD) or short message service (Short Message Service, SMS).
Considering the SCC AS in the ICS needs to implement an interworking operation between the I1 protocol and the Session Initiation Protocol (Session Initiation Protocol, SIP), the I1 protocol may use most of concepts or terms of the SIP. To make a difference, an I1 prefix is usually added before the concepts or terms of SIP to indicate the concepts or terms of the I1 protocol.
Because there are a large number of IMS sessions, the I1 protocol, as an IMS session control protocol, also has a large traffic volume (that is, overheads generated in the transfer of I1 protocol messages). Various applications or value-added services are already developed on the basis of the USSD or the SMS. Therefore, if the I1 protocol uses the USSD or the SMS as the transmission protocol, traffic of the USSD or the SMS may be greatly affected. Therefore, the ICS working group requires that the message overheads of the I1 protocol should be minimized during the design.